Abstract: The evolution of content， morphology and chemical composition of nonmetallic inclusions were researched based on sampling during IF(interstitial free) steel production process of the DSLDRHCC route and the analysis of total oxygen content， nitrogen content， the morphology of the nonmetallic inclusion and composition， combing with the thermodynamic analysis. Results showed that the total oxygen content decreased during steel smelting process， but the oxidizability of ladle slag must be controlled within lower level to further decrease the total oxygen content in the casting. Protective casting technique should be optimized to control the significant increase of the nitrogen content during continuous casting process. During the RH refining process， pure cluster Al2O3 inclusions formed after the deoxidation， spherical complex inclusions of AlTiO and dual layer AlTiCaMgO formed after the alloying of titanium. During the solidification of the steel， dual layer complex inclusions were formed consisting of TiAlMnOS core surrounded by TiN and/or AlN. Large scale complex inclusions were also existed in the casting with the main composition of TiAlCaSiMnO containing some potassium and sodium from the slag entrapment of mold fluxes.

Key words: IF steel, steel smelting process, total oxygen content, nonmetallic inclusions, evolution